1. The Field of the Invention
This invention relates to apparatus and methods for dampening vibrations during rotary drilling operations, and, more particularly, to a novel apparatus and method for dampening both axial and torsional vibrations during the operation of a top-drive rotary blasthole drill.
2. The Prior Art
For a number of reasons, it is often necessary or desirable to drill into the earth's surface. Such drilling operations are, for example, one of the principal means by which petroleum and natural gas products are made available for use. Similarly, it is usually necessary to drill into the earth's surface in order to recover fuels from oil shale formations. Likewise, drilling operations are frequently used to both locate and extract various mineral and water deposits.
One of the most common tools for drilling into the earth's surface is a rotary blasthole drill. Such an apparatus comprises a large rig, to which is attached a rotary drive mechanism. Importantly, the drill's rotary drive is capable of being raised and lowered along a substantially vertical axis directly above the formation to be drilled. Additionally, a length of drill pipe is connected to the rotary drive so as to extend downwardly therefrom in a substantially vertical direction; and a drill bit is secured to the downward end of the drill pipe.
When using the rotary blasthole drill apparatus described above, the drill's rotary drive is first activated so as to rotate both the drill pipe and the drill bit at the desired speed. Then, the rotary drive, together with the drill pipe and bit, is lowered, such that the drill bit contacts the surface of the formation to be drilled. A predetermined amount of downward pressure is then continuously applied to the rotating drill pipe and bit, thereby forcing the drill bit to cut downwardly into the formation. Throughout the drilling operation, air is forced through the drill pipe, thereby forcing debris out of the hole and maintaining a clean surface upon which the drill bit may operate.
As soon as the drilled hole is deep enough to accommodate the first length of drill pipe, the drill's rotary drive is disconnected from the drill pipe and raised to its original position. A second length of drill pipe is then connected between the rotary drive and the first length of drill pipe, thereby increasing the effective length of the drill. Thereafter, the rotary drive is again activated, and the drilling operation is continued. This procedure is then repeated until the desired hole depth is achieved.
One of the major problems encountered during rotary blasthole drilling operations is the generation of axial (vertical) and torsional (rotational) vibrations. Generally speaking, these vibrations are simply due to the rotation of the drill and the simultaneous action of the drill bit cutting through the formation. However, it has also been noted that the magnitude of these vibrations is somewhat dependent upon the particular type of formation being drilled. For example, large vibrations are typically generated almost continuously when drilling through extremely hard formations, or when drilling through fractured or layered formations. Similarly, large vibrations are usually generated as the drill bit travels out of a hard formation and into a softer formation, or vice versa.
Vibrations during the operation of a drill may cause difficulties in several ways. First, such vibrations may damage the drill, in that various rigid components of the drill may develop fatigue cracks as a result of prolonged subjection to the vibrations. In addition, even if the drill is not damaged, the vibrations may dramatically increase the wear on the drill bit, thereby reducing the drill bit's useful life. Also, the vibrations may cause large fluctuations in the downward pressure which is applied to the drill, thereby causing the drill bit to bounce or hop on top of the formation being drilled. Significantly, due to the hopping of the bit, the drill may cut slowly and unevenly through the formation. It will be readily appreciated that vibrations also greatly increase the operator noise of the drill. Further, in order to minimize the above-mentioned difficulties, it is often necessary to reduce the drilling speed. Importantly, all of these difficulties may significantly increase both the time needed for drilling and the costs of the drilling operation.
In an effort to eliminate the above-identified problems, those skilled in the art have developed various devices to dampen vibrations during the operation of a rotary drill. Typically, these devices comprise some type of shock absorber which is connected between the drill's rotary drive and the drill pipe. Often, the shock absorber in these devices includes some type of resilient material which absorbs the vibrations and dissipates the energy thereof as heat.
One such prior art dampener comprises two parallel, horizontal plates. One of the plates is connected, either directly or indirectly, to the rotary drive and the other plate is connected to the drill pipe. This apparatus further comprises a resilient pad which is bonded between these two plates, and the entire apparatus has a hole through the center thereof in order to accommodate the air which is forced through the drill pipe.
This type of prior art vibration dampener quite adequately dampens axial (vertical) vibrations. It will be appreciated that, in response to axial vibrations, the two plates of the apparatus are forced closer together. As a result, the resilient pad between the two plates is compressed, thereby dissipating the energy of the axial vibrations as heat.
On the other hand, however, it has been found that this type of prior art vibration dampener does not respond adequately to torsional (rotational) vibrations. When this device is subjected to torsional vibrations, a shearing force is exerted upon the bond between the resilient pad and the two plates. Consequently, over a period of time or in response to large torsional vibrations, the bond between the resilient pad and one or both of the plates is frequently broken. This makes further rotation of the drill virtually impossible and also makes it extremely difficult to thereafter lift the drill pipe out of the hole. In an effort to minimize such difficulties, these prior art dampeners are often provided with flexible safety straps connected between the two plates. Although such straps may be helpful, these safety straps are also frequently broken or damaged due to stress or to contact with tools or other objects.
A second type of prior art vibration dampener likewise comprises two parallel, horizontal plates, one of which is connected to the drill's rotary drive, either directly or indirectly, and the other of which is connected to the drill pipe. These two plates are connected by a plurality of bonded segments, the segments being attached to the plates so as to define a cylinder which is substantially concentric with the drill pipe. Each bonded segment comprises two, rigid lugs, one of which is positioned upwardly and the other, downwardly; and the two lugs are each bonded to opposite sides of a strip of resilient material. Importantly, these bonded segments are attached to the two plates such that a counterclockwise rotation of the lower plate with respect to the upper plate causes the resilient strip of each bonded segment to be compressed by the two adjacent lugs.
This second type of prior art vibration dampener very adequately dampens torsional (rotational) vibrations. Since the drill rotates in a clockwise direction, torsional vibrations typically exert a counterclockwise force on the drill pipe, thereby causing the lower plate of the dampener apparatus to be rotated slightly in a counterclockwise direction with respect to the upper plate. As mentioned above, such rotation causes the resilient strip of each bonded segment to be compressed, thereby dissipating the energy of the torsional vibrations as heat.
In spite of its ability to dampen torsional vibrations, however, it has been found that this second type of prior art vibration dampener does not adequately respond to axial vibrations. Axial vibrations exert a sheering force on the bond between the resilient strip and the two lugs of each bonded segment. Consequently, due to prolonged use of the apparatus or to large axial vibrations, the bond in one or more of the bonded segments is frequently broken. Therefore, as with the first type of prior art dampener described above, this dampener also requires some kind of safety feature in order to permit the drill pipe to be thereafter rotated and/or lifted.
In addition to the above-mentioned problems, there are several significant problems which are common to both of these prior art vibration dampeners. First, the resilient members in both of these dampeners are typically exposed to dirt and the elements. Such exposure causes accelerated wear of these resilient members and significantly reduces their useful life. In addition, tools and other objects may easily come into contact with the exposed, resilient members, thereby damaging such members and reducing the effectiveness of the vibration dampener. Further, these prior art dampeners typically include a plurality of bolts or screws whose heads lie against the top and/or bottom of the dampener. These bolts or screws must be frequently checked and tightened in order to prevent them from coming loose during operation of the drill. In addition, the bolts or screws are often damaged due to contact with the deck platform of the drilling rig.
It will be readily appreciated that the problems discussed above may give rise to a number of undesirable consequences. First, the prior art vibration dampeners tend to wear out within a relatively short time, thus necessitating frequent repair and/or replacement. Of course, this may substantially increase the costs of the drilling operation. In addition, since both the weight of the drill pipe and the force of the vibrations tend to increase with hole depth, the structural instability of the prior art devices places a natural limit on the maximum drilling depth which can be attained. Further, the prior art devices may prove somewhat dangerous since, when the prior art vibration dampeners become fatigued or damaged, some of the component parts may fall from the device and injure nearby workers.
Accordingly, it would be an improvement in the art to provide an apparatus and method which is effective for dampening both axial and torsional vibrations which are generated during rotary drilling operations. It would also be an improvement in the art to provide a vibration dampener wherein there is no possibility of breaking a bond between a resilient member and a rigid member. Additionally, it would be an improvement in the art to provide a vibration dampener which would permit the drill pipe to be rotated and lifted even after the dampening member is completely fatigued. Further, it would be an improvement in the art to provide a vibration dampener wherein the resilient dampening members are not exposed to dirt or the elements or to contact with tools or other objects. It would also be an improvement in the art to provide a vibration dampener which does not include bolts or screws which may come loose or be damaged during operation of the drill. Such a device and method is disclosed and claimed herein.